As a conventionally employed battery case lid of this type, a configuration disclosed in Patent Document 1 and so on, for example, may be cited. FIG. 6 is an exploded perspective view showing a battery case using a conventional lid. In the drawing, a lid 1 together with a case main body 2 having a closed-end cylindrical shape constitutes a battery case 3. The lid 1 and the case main body 2 are formed from metal plate made of aluminum, an aluminum alloy, nickel-plated steel, stainless steel, or the like. The lid 1 is provided with a lid main body 4 and a safety valve 5. The lid main body 4 is a planar portion formed in a rectangular shape having a short side 40 and a long side 41 when seen from above. The safety valve 5 is provided to prevent the battery case 3 from bursting open when internal pressure of the battery case 3 (referred to hereafter as case internal pressure) rises. When the internal pressure of the battery case 3 exceeds a predetermined value, the safety valve 5 ruptures, thereby releasing the internal pressure of the battery case 3 to the outside. The safety valve 5 includes an annular thin portion 50 formed integrally with the lid main body 4 by performing a coining (pressing) process in which an marking punch is pressed against the metal plate serving as a base material of the lid 1. When the internal pressure of the battery case 3 exceeds the predetermined value, all or a majority of the annular thin portion 50 fractures, thereby rupturing the safety valve 5.
Patent Document 1: Japanese Patent Application Publication No. 2000-285892